Candidiasis encompasses fungal infections caused by a variety of species of the genus Candida, in particular mostly by Candida albicans, which is a yeast-like fungus. Candida spp. are normally present in the mouth, vagina, and intestines of healthy individuals; normal bacteria in these areas keep the amount of Candida spp. in check. Infection by candidal fungi normally depends on a weakened immune status of an individual to invade tissue that normally would be resistant to infection and the opportunity to gain access to the circulatory system. Candida infections that develop in immunocompromised individuals can affect the entire body (e.g., disseminated or systemic candidiasis) and may become life threatening. The most common condition is topical candidiasis (fungus growing on the surface of the body). An example of this is a common form of “diaper rash” in infants. Topical candidiasis can affect the skin, the vagina, the mouth (e.g., oral candidiasis) and the esophagus, and in immunocompromised individuals (e.g., HIV patients).
Candida infections are opportunistic and generally begin with increased colonization of the junction of mucous membranes and skin surfaces of vulnerable parts of the body such as, for example, oral, nasal, vaginal, and anal orifices, and the lining of the respiratory tract. Under some abnormal conditions, including the reduction of normal bacteria in a particular part of the body or skin defects such as wounds, ulcerations, and burns, the fungi can overgrow and cause infection of the outer layers of the skin and mucous membranes. This may occur in the mouth (oral thrush), in the vagina or penis (genital candidiasis), between folds and surfaces of skin (intertrigo), and in and around the nails (paronychia and onychomycosis).
In some instances, the fungus enters the bloodstream and causes disseminated disease affecting internal body organs such as the kidneys, spleen, lungs, liver, eyes, meninges, brain, and heart valves. This condition is called systemic or disseminated candidiasis; it can result in a range of diseases such as superficial mucocutaneous disease, candidiasis of the liver and spleen (hepatosplenic candidiasis), and peritonitis. This is usually seen in patients who are seriously ill with other diseases who have been receiving potent antibiotics that treat bacterial infection.
Oral candidiasis (sometimes referred to as “thrush”) is an infection in which the fungus of the genus Candida (a yeast) accumulates on the mucous membranes of the mouth. It is most often caused by Candida albicans, or less commonly by Candida glabrata or Candida tropicalis. If occurring in the mouth of a baby, the candidiasis is commonly referred to as oral thrush, whereas if occurring in the mouth or throat of an adult, it may also be termed candidosis or moniliasis.
Oral infection by Candida species may not be immediately noticeable but can develop suddenly and may persist for a long time. The infection usually appears as thick white or cream-colored deposits on mucosal membranes such as the tongue, inner cheeks, gums, tonsils, and palate. The infected mucosa may appear inflamed (red and possibly slightly raised) and sometimes have a cottage cheese-like appearance. The lesions can be painful and may become tender and often bleed if rubbed or scraped. Cracking at the corners of the mouth, a cottony-like sensation inside the mouth, and even temporary loss of taste can occur. In more severe cases, the infection can spread down the esophagus and cause difficulty swallowing, which is sometimes referred to as esophageal candidiasis. Thrush does not usually cause a fever unless the infection has spread beyond the esophagus to other body parts, such as the lungs (i.e., systemic candidiasis).
Although oral thrush can affect anyone, it is more likely to occur in babies and in people who wear dentures, use inhaled corticosteroids, or have compromised immune systems. Oral thrush and other Candida infections can occur when the immune system is weakened by disease or drugs such as prednisone, or when antibiotics disturb the natural balance of microorganisms in the body. Normally, the immune system repels harmful invading organisms, such as fungi, while maintaining a balance between “good” and “bad” microbes that normally inhabit the body. When these protective mechanisms fail, however, an oral thrush infection may take hold.
The following diseases or conditions may make one more susceptible to oral thrush infection:
1) HIV/AIDS. The human immunodeficiency virus (HIV) damages or destroys cells of the immune system, making one more susceptible to opportunistic infections. Repeated bouts of oral thrush may be the first sign of an HIV infection.
2) Cancer. The immune system is likely to be weakened from the disease and from treatments, such as chemotherapy and radiation. Both the disease and treatments can increase the risk of Candida infections such as oral thrush.
3) Diabetes mellitus. In untreated or under-treated diabetes, the saliva may contain large amounts of sugar, which encourages the growth of Candida. 
4) Vaginal yeast infections. Vaginal yeast infections are caused by the same fungus that causes oral thrush. Although a yeast infection is not typically dangerous, a pregnant female can pass the fungus to the baby during delivery.
Oral candidiasis can be treated with topical anti-fungal drugs, such as nystatin, miconazole, Gentian violet, or amphotericin B. Topical therapy is normally provided as an oral suspension which is washed around the mouth and then swallowed by the patient. Patients who are immunocompromised, either with HIV/AIDS or as a result of chemotherapy, may require systemic treatment with oral or intravenous administered anti-fungals. Some anti-fungal medications, however, may cause liver damage. For this reason, a physician will likely perform blood tests to monitor liver function, especially if prolonged treatment is required or there is a history of liver disease.
Some Aspergillus species cause serious disease in humans and animals. The most common pathogenic species include Aspergillus fumigatus and Aspergillus flavus. Aspergillus flavus produces aflatoxin which is both a toxin and a carcinogen, and which can contaminate foods. The most common causing allergic disease are Aspergillus fumigatus and Aspergillus clavatus. Other species, Aspergillus spp., are important agricultural pathogens.